If your side wall zips from the ground, up the end wall and along the roof I would be inclined to just vip it part way then fold over and roll up back as has as you have it zipped up.
You could sew some ties in the appropriate spots on the inside and outside of the wall, to secure the rolled back section.
That would give you the best of both worlds, as well as utilising your existing side wall AND saving on cost, storage, & weight of an additional; 1/2 wall.
I always prefer multiple use to additional stuff.